The present invention relates to a process for manufacturing a semiconductor integrated circuit device and, more particularly, to a technique which is effective when applied to a gas-phase treating process for a semiconductor wafer using a hydrogen gas.
In a semiconductor manufacturing process, it is known to use a hydrogen-annealing treatment for feeding hydrogen to a polycrystalline silicon film for constructing the gate electrodes of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a Si (silicon) substrate. For this hydrogen annealing treatment, the hydrogen gas is introduced into a batch type or sheet type hydrogen-annealing furnace housing a semiconductor wafer to heat-treat this semiconductor wafer. In a hydrogen atmosphere at about 400.degree.. After this hydrogen-annealing treatment, the trap level, as caused by the dangling bonds (or uncoupled bonds) of Si, is terminated by the hydrogen so that the characteristics of the MOSFET are improved.
In the prior art, the hydrogen gas, as discharged from the hydrogen-annealing furnace, is eliminated by a combustion method. In this regard, air is introduced into the exhaust line of the hydrogen-annealing apparatus to burn and convert the hydrogen into water by the spark ignition method. In a hydrogen-annealing furnace of a relatively small scale, on the other hand, the discharged hydrogen may be diluted with a large amount of nitrogen gas or air and released to the atmosphere.
In a process for forming MOSFETs over a Si substrate, on the other hand, the Si substrate is wet-oxidized to form a gate oxide film on its surface. For this formation, there is also utilized a combustion method, in which water is generated by burning hydrogen in an oxygen atmosphere and the water is fed together with the oxygen to the surface of the semiconductor wafer.
For generating a water/hydrogen mixed gas to wet-oxide the Si substrate, a catalytic method is well known in the art in addition to 25, the combustion method. For example, Japanese Patent Laid-Open No. 5-152282/1993 (hereinafter to be called "Ohmi") has disclosed a thermal oxidizing apparatus in which a hydrogen gas inlet pipe is made at its inner face of Ni (nickel) or a Ni-containing material and which is equipped with means for heating the hydrogen gas inlet pipe. This thermal oxidizing apparatus generates water by bringing the hydrogen into contact with the Ni (or Ni-containing material) in the hydrogen gas inlet pipe heated to 300.degree. C. or higher, to generate hydrogen activated species and by causing these hydrogen activated species and the oxygen (or the gas containing oxygen) to react with each other.
In a process for forming gate electrodes over the gate oxide film thus formed by the wet-oxidizing method, moreover, the gate electrode material, as deposited on the gate oxide film, is patterned by a dry-etching method. After this, the photoresist employed as the etching mask is eliminated by aching treatment, and the dry-etching residue or aching residue, as left on the surface of the substrate, is eliminated by an etching liquid, such as hydrofluoric acid.
With this wet-etching treatment, the gate oxide film is etched off not only from regions other than those under the gate electrodes, but also isotropically from the end portions of the side walls of the gate electrodes, thereby to cause an under-cut. This under-cut will cause, as it is, a defect in the form of a drop in the withstand voltage of the gate electrodes. In order to improve the profile of the under-cut end portions of the side walls of the gate electrodes, therefore, a so-called "light oxidation treatment" is performed to thermally oxidize the substrate again, thereby to form an oxide film on its surface.
If, however, the light oxidation treatment is applied to the gate electrodes of the poly-metal structure, which contains a refractory metal, such as W (tungsten) or Mo (molybdenum), that is liable to be oxidized in a hot oxygen atmosphere, the refractory metal film is oxidized to raise its resistance or is partially separated from the substrate. For a gate treating process using a poly-metal, therefore, there are required counter-measures for preventing the refractory metal film from being oxidized during the light oxidation treatment.
Japanese Patent Laid-Open No. 59-132136/1984 (hereinafter to be called "Kobayashi") has disclosed a technique for oxidizing Si selectively, but not a W (or Mo) film, by light-oxidizing the gate electrodes of the poly-metal structure including the W film or the Mo film over the Si substrate, in a mixed atmosphere of steam and hydrogen.
This technique utilizes the fact that the partial pressure ratio of steam/hydrogen for balancing the oxidizing/reducing reactions is different between W (Mo) and Si. The selective oxidation of Si is realized by setting the partial pressure ratio within a range wherein the W (Mo) may be oxidized with steam, but quickly reduced with the coexisting hydrogen, whereas the Si may be left oxidized. On the other hand, the steam/hydrogen mixed atmosphere is generated by the bubbling method of feeding the hydrogen gas to pure water contained in a container, and the steam/hydrogen partial pressure ratio is controlled by changing the temperature of the pure water.
In the light oxidation process, as disclosed, the Si substrate is oxidized with the steam/hydrogen mixed gas so that the hydrogen gas is contained in the exhaust gas discharged from the oxidizing furnace. In this case, too, there are required some counter-measures for eliminating the hydrogen gas from the exhaust gas.
Another well-known exhaust gas eliminating method, as employed in the semiconductor manufacturing process, is disclosed in Japanese Patent Laid-Open No. 8-83772/1996 (hereinafter to be called "Watanabe"). In this method, an exhaust gas containing tetraethoxysilane discharged from a CVD (Chemical Vapor Deposition) apparatus is guided into an adsorption tower to come into contact with a metal oxide catalyst (or an adsorbent carrying the catalyst) so that it may be oxidized and decomposed into a powder of silicon dioxide and diethyl ether. The metal oxide catalyst to be employed is exemplified by NiO, CuO, Mn.sub.2 O.sub.3 or Fe.sub.2 O.sub.3.
Moreover, Japanese Patent Laid-Open No. 9-75651/1997 (hereinafter to be called "Koyashiki") has disclosed a method in which a silane gas is eliminated as the powder of silicon dioxide by mixing and oxidizing an (water-soluble) ammonium gas and the silane gas with oxygen, in which the ammonia gas is eliminated by rinsing and in which the remaining noxious gases are adsorbed and eliminated with active carbon.